In a television receiver, it is often desirable to use non-linear signal processing in order to improve the subjective appeal of displayed images. So called "black stretch" and "white stretch" circuits are used to improve the image contrast ratio by adaptively altering the shape of a signal transfer function in the dark and bright image areas, respectively. So called gamma correcting circuits also alter a signal transfer function in a non-linear manner, either statically or dynamically, to compensate for differences between the non-linear characteristics of television cameras utilized in broadcast studios and the non-linear characteristic of the display device of a receiver. So-called "auto-pedestal" circuits are also used to adaptively adjust the brightness of a displayed image by inserting a "blacker-than-black" variable amplitude pulse during the back- porch region of the luminance signal. The brightness of a displayed image is altered because the "auto-pedestal" function changes the relationship between the clamping level of a "back-porch" clamp and the level of the video signal which is clamped. If the synchronization component and the image component of a video signal are processed together, each of these "picture enhancement" circuit techniques may impact the ability of a synchronizing pulse separator to distinguish between the "blacker-than-black" sync pulses and the image portion.
FIG. 1 shows a typical application of a non-linear "picture enhancement" circuit. In this example, a television signal received by an antenna 60 is tuned by a tuner 65 and demodulated by IF section 70 to produce a baseband video signal 75. This signal is separated into a composite luminance (luma) signal 104, containing both image and sync pulse components, and a chrominance (chroma) signal 85 by a luma-chroma separator 90. The chroma signal is processed by processing circuits 80 to produce color difference signals 86. Color difference signals 86 are matrixed with a luma signal 87 produced by a luma processor 81 in luma-chroma matrix 82 to produce primary color signals 88 suitable for application to a picture tube 83. A non-linear "picture enhancement" circuit 110 precedes luma processing circuit 81 and supplies to it a non-linearly processed luma signal 105. Composite luma signal 104 produced by lumachroma separator 90 is coupled in parallel fashion to picture enhancement circuit 110 and a sync separator 100. Separated sync signal 115 is applied to horizontal and vertical scan processing circuit 120, which in turn provide deflection signals 89 to a deflection unit 84.
The arrangement shown in FIG. 1 has the desirable feature that sync separator 100 derives the composite sync signal from composite luma signal 104 before it is processed by non-linear picture enhancement circuit 110. A similar arrangement is disclosed with respect to FIG. 12 of U.S. Pat. No. 4,489,349 issued to Okada on Dec. 18, 1984 and assigned to the Sony Corporation.
As shown in FIG. 2A, the use of a picture enhancement circuit in conjunction with a combined television processor integrated circuit (IC), often called a "one-chip" television IC, such as the Toshiba TA8680 necessitates that the input signal to sync separator 100 be derived from the output signal of picture enhancement circuit 110. This is due to the fact that in ICs such as the TA8680, the inputs to sync separator 100 and luma processing circuit 81 are connected together within the IC and are not accessible separately.
An example of an IC providing "black-stretch" and "auto-pedestal" functions, which may be used in the arrangements shown in FIGS. 1 and 2A is the Sony CX20125 dynamic picture IC. As shown in FIG. 2B, the CX20125 IC receives a composite luminance signal 104, a composite horizontal and vertical retrace blanking signal 101 and a horizontal "back-porch" clamping pulse signal 102 at respective inputs. In response, it provides a composite luminance signal 105 which has been non-linearly processed in accordance with a "black-stretch" function. In addition, an "auto-pedestal" pulse is added to the "back-porch" of the composite luminance signal. The clamping pulse signal is used to provide back-porch clamping for its own signal processing as well as to aid in generation of the "auto-pedestal" pulse. The CS20125 IC uses the composite retrace blanking signal to inhibit black-stretch during horizontal and vertical blanking intervals. Such retrace blanking interval inhibiting provisions for a non-linear processing system are also disclosed with respect to FIG. 11 of the aforementioned Okada patent.
To understand how a problem in deriving the composite sync signal arises when the arrangement of FIG. 2B is used, it will be helpful to review the operation of separating the composite sync pulses from the composite luminance signal. Reference will be made to FIG. 3 during this description. The typical syn separator arrangement includes a clamp 103 to restore the DC level of the composite luma signal, after it is AC coupled through a capacitor (not shown), by clamping the peaks of the sync pulses to a reference voltage 106. The signal 107 so restored is applied to a level comparator 108 having a reference voltage 109 which is related to reference voltage 106 of clamp 103. By choosing reference voltage 109 of comparator 108 to be at an intermediate level between the "back-porch" level and the expected sync tip level, a composite horizontal and vertical sync pulse signal without any artifacts of the image related video signal will be produced at an output 115 of comparator 108.
The horizontal and vertical portions of a typical composite luminance signal are shown in FIGS. 4a and 4b, respectively. Image portion 280 of the composite luminance signal extends from blanking level 204 to peak white level 205, while horizontal sync pulses 206 and the vertical sync pulses 207 extend below blanking level 204 to sync tip level 208. The NTSC television standard specifies that the amplitude between blanking level 204 and sync tip level 208 should be 40% of the amplitude between blanking level 204 and peak white level 205. This sync amplitude relationship provides adequate margin for any inaccuracy of the sync separator and allows the sync component to be reliably separated from the image component.
The horizontal and vertical portions of a composite luminance signal which have been processed by a non-linear processing IC such as the the CX20125 are shown in FIGS. 6a and 6b, respectively. In contrast to the waveform of FIG. 4a, note that the waveform of FIG. 6a has a pulse 211 inserted during the back-porch interval following horizontal sync pulse 206. This inserted pulse is a variable amplitude "auto-pedestal" pulse. The amplitude of the auto-pedestal pulse typically varies from blanking level 204 to a maximum level 212. Level 212 is about 50% of the amplitude between sync tip level 208 and blanking level 204. The waveform shown in FIG. 4a includes a dark portion 210 during active scan time. This dark image region extends to a level 209, which is "whiter" than the "blacker-than-black" blanking level 204. If the total time duration of dark image regions is relatively large during a field, "black-stretch" processing will extend level 209 to a black level or even to blacker-than-black blanking level 204. For relatively short durations, "black-stretch" processing will extend dark level 209 to a level 213 below blanking level 204 as shown in FIG. 6a.
The horizontal and vertical retrace blanking intervals are shown in FIGS. 5a and 5b, respectively. Comparing the waveforms of FIGS. 4a and 5a, it will be noted that the horizontal blanking interval 214 of the received signal and the retrace blanking interval 216 are approximately coincident. However, comparing the waveforms of FIGS. 4b and 5b, it will be noted that vertical retrace blanking interval 217 is considerably shorter than the vertical blanking interval 215 of the received signal. This is a result of typical receiver design practice since it allows for a more economical structure and usually causes no problem. However, in receivers using IC such as the CX20125, the short vertical retrace blanking interval results in the response shown in FIG. 6b because "black-stretch" processing is not inhibited during interval 218 between the beginning of blanking interval 215 of the received signal and the beginning of retrace blanking interval 217. As shown in FIG. 6b, the horizontal trace portions which occur during interval 218 may be stretched from their normal level 204 to the blacker-than-black level 213. In a similar manner, the vertical equalizing pulses 282 which occur during interval 218 may be stretched from level 208 to level 281. In this regard, it is noted that while the equalizing pulses are stretched, the horizontal pulses occurring during interval 218 are not stretched because "black-stretch" processing is inhibited during horizontal blanking intervals. Just as black region 210 shown in FIG. 4a is sometimes stretched depending on the total duty cycle of black image regions, the horizontal trace regions which occur during interval 218 may be kept at blanking level 204 or extended towards maximum extension levels 213 or 281 depending upon variations of scene content. Thus, the margin for setting the sync separator comparator reference level is between sync tip level 208 and the lower of levels 212 or 213 for horizontal sync pulses, and is not predictable for vertical sync pulses. It is thus difficult to set a reliable comparator reference.